1. Field of the Invention
The present invention relates to an indicating instrument, and more particularly to a motor controller for controlling a stepping motor used in the indicating instrument, a semiconductor integrated circuit used in the motor controller and a method for controlling the stepping motor.
2. Description of the Related Art
Since a stepping motor operates with high precision and is inexpensive, the stepping motor is widely used in an indicating instrument such as a speedometer for displaying the speed of an automobile and a tachometer for displaying the number of revolutions of an engine. In the indicating instrument such as the speedometer and the tachometer, a pointer of the indicating instrument may be incapable of a precise indication due to vibrations of a vehicle, step-out of the stepping motor and the like. Therefore, for example, in the speedometer, a zero position bar for positioning the pointer to indicate zero (hereinafter simply referred to as “a zero position”), that is, for fixing the pointer at a position indicating that the speed per hour is 0 Km/h. By measuring voltage values of induced electromotive forces generated by the stepping motor, both when the pointer of the stepping motor makes contact with the zero position bar and when the pointer does not make contact with the zero position bar, a technique for detecting the zero position is known.
The induced electromotive forces are measured at a drive angle at which the stepping motor does not need to be driven. Moreover, one end of an inductor in the stepping motor is configured to go to a high-impedance state during the driving of the stepping motor, and the voltage value derived from the induced electromotive force is measured after generation of a counter electromotive force. Accordingly, it is impossible to detect the zero position while a drive angle requiring the driving of the stepping motor and the counter electromotive force is generated. As described above, because conditions in which the zero position can be detected are limited, it has been difficult to detect the zero position with high precision.